The development of methods and apparatus to detect small molecules using field-portable instrumentation is the ultimate goal of substantial research in the field of chemical analysis. In a research laboratory, small molecules are typically detected by gas chromatography (GC), liquid chromatography (LC), mass spectrometry (MS), or both (GC-MS, LC-MS). Despite intense effort, including efforts by the Defense Advanced Research Projects Agency (“DARPA”), over the past decade, miniaturization of this type of laboratory instrumentation while maintaining acceptable mass resolution and absolute sensitivity has proven impossible.
Field portable apparatus and methods for the detection of large molecules or proteins, cells, and DNA is possible. With respect to these large species, naturally-occurring detection molecules exist (such as antibodies or complementary sequences), to which detection tags (mostly optical) can be attached. Thus, 99% of all bioassays for these relatively large species involve a “sandwich” format, in which the analyte is immobilized by non-covalent interaction with a capture molecule, and quantified by non-covalent interaction with a labeled detection molecule.
Harnessing a New Detection Modality
Until recently, the only available optical detection tags were based on fluorescence. In U.S. Pat. No. 6,514,767, which patent is incorporated herein by reference in its entirety, Applicants described an optical detection tag based on surface enhanced Raman scattering (“SERS”). A tag consistent with the U.S. Pat. No. 6,514,767 patent is depicted in FIG. 1.1 With SERS, molecules in very close proximity to nanoscale roughness features on noble metal surfaces (typically gold, silver or copper) or suitably-sized metal nanoparticles give rise to million- to trillion-fold increases [known as enhancement factor (EF)] in scattering efficiency.2 With these tags, the SERS signal comes from submonolayers of reporter molecules sandwiched between the noble metal and a glass shell. In typical assays, the glass surface is coated with a biofuctional species that attaches to a bioanalyte of interest (e.g. an antibody for protein detection). These particles offer several significant advantages as optical detection tags: (i) they are excited in the near-infrared, eliminating the background visible fluorescence signal invariably associated with real-world measurements; (ii) different reporter molecules give rise to unique, narrow spectral features, allowing multiple signatures to be simultaneously detected; (iii) portable, robust and inexpensive instrumentation amenable to point-of-use implementation already exists; and (iv) exceptional sensitivity is possible.
Unfortunately, the SERS nanotags of FIG. 1 cannot be leveraged for detection of chemical warfare agents, e.g. sarin, because pairs of capture/detection molecules do not exist for low-molecular weight species, and more importantly, the Raman signal is (by design) locked in via the incorporation of a reporter molecule. Yet sarin and other low molecular weight species each have a distinctive SERS spectrum that could serve as a basis for ultrasensitive, accurate detection.
The present invention is directed toward overcoming one or more of the problems discussed above.